In recent years, mobile equipments such as cellular phones have been spread, and semiconductor devices as the main components of personal computers have had higher speed and higher frequency. Accordingly, the multilayer ceramic capacitors mounted on these electronics equipments are increasingly required to become compact high capacity for use in power sources, and the dielectric layers constituting the multilayer ceramic capacitors are required to become thinner and highly laminated.
Conventionally, dielectric constant materials composed mainly of barium titanate have been used as a dielectric ceramics for a dielectric layer constituting a multilayer ceramic capacitor. Recently, the dielectric ceramics has been developed in which oxide powders such as magnesium, rare earth element and vanadium are added to barium titanate powder, and the magnesium and the rare earth element are dissolved to form a solid solution in the vicinity of the surfaces of crystal grains composed mainly of barium titanium. The above dielectric ceramics has been practiced as a multilayer ceramic capacitor (refer to, for example, patent document 1 and patent document 2).
For example, in the patent document 1, attempts to improve characteristics such as dielectric breakdown voltage and IR accelerated aging are made by incorporating magnesium, rare earth element and vanadium into barium titanate as the main ingredient of the crystal grains constituting the dielectric layer as described above, thereby obtaining the crystal structure (so-called core-shell structure) in which the diffraction line of (200) plane and the diffraction line of (002) plane are partially overlapped with each other in an X-ray diffraction chart, resulting in a wide diffraction line.
Also in the patent document 2, attempts to improve life characteristics are made by forming the core-shell structure having a shell phase that has a proper concentration gradient of vanadium within crystal grains. That is, the excessive diffusion of vanadium into barium titanate and the deposition of a vanadium compound are suppressed, while suppressing the movement of electrons existing in the crystal grains by adjusting the valiance of vanadium dissolved in the barium titanate to form a solid solution, into a range around tetravalent.
As used herein, the term “core-shell structure” of the crystal grains refers to the structure that a core part as the central part of the crystal grains and a shell part as the outer shell part thereof form physically and chemically different phases. The crystal grains composed mainly of barium titanate exist in the state in which the core part is occupied by barium titanate having a tetragonal crystal structure, and the shell part is occupied by the barium titanate having a cubic crystal structure.
Patent document 1: Japanese Unexamined Patent Application Publication No. 8-124785
Patent document 1: Japanese Unexamined Patent Application Publication No. 2006-347799